1. Field of the Invention
The present invention relates to a measuring tool for measuring a displacement of a movable member relative to a stationary member.
2. Description of Related Art
Calipers, micrometers, indicators, test indicators, height gauges, and scales are known as digital measuring tools for measuring dimensions and the like of an object. Such measuring tools are provided with a stationary member, a movable member arranged movable relative to the stationary member, a detector for detecting a displacement of the movable member relative to the stationary member, and a display for displaying the displacement of the movable member relative to the stationary member detected by the detector.
For example, as shown in FIG. 1, a test indicator 1 includes: a body 10; a stylus 11 penetrating the body 10 and supported swingable around a supported portion 111; an encoder 12 having an electrode 121 provided to a closer end to the body 10 of the stylus 11 and a detector head 122 which is provided to the body 10 and electrostatically coupled with the electrode 121 to detect a displacement of the stylus 11 relative to the body 10; a display 13 for digitally displaying the displacement of the stylus 111 relative to the body 10 detected by the encoder 12; and an operating section 15 for a so-called preset operation.
The test indicator 1 arranged such is a measuring tool for measuring a height and the like of an object 2 from a displacement of the stylus 11 obtained when a contact portion 112 provided to an end of the stylus 11 is contacted to the object 2.
The test indicator 1 can also work out a comparison and the like of a plurality of objects by carrying out the preset operation.
In order to compare heights of two objects, firstly, the contact portion 112 of the stylus 11 is brought into contact with an upper surface of one object, and the operating section 15 is operated. The displacement then of the stylus 11 is stored as a measurement reference value. Next, the contact portion 112 of the stylus 11 is contacted to the upper surface of the other object. Consequently, the difference between the displacement of the stylus 11 at this time and the measurement reference value previously stored is displayed on the display 14, thereby enabling comparison between the heights of the two objects.
As an example of such an indicator, there is disclosed a test indicator which has a detector that electrically senses a moire fringe generated by a displacement of a stylus enabling a precise measurement (Patent Document: JP-UM-B-02-45762).
However, a conventional test indicator such as one in Patent Document can not precisely determine a measurement reference value since an operating section for handling a preset operation is provided to the body.
This problem will be described with reference to FIGS. 5 to 7.
In FIGS. 5 to 7, the horizontal axis represents time, and the vertical axis represents a displacement of the stylus. The curve shows the displacement of the stylus that occurs during the preset operation. An operation state and a stored measurement reference value are illustrated in a timing chart under the curve.
FIG. 5 shows a displacement of the stylus 11 during the preset operation with a conventional test indicator 1.
A preset operation starts with bringing the stylus 11 into a stationary state by, for example, contacting the contact portion 112 of the stylus 11 to the object. The stationary state marks time point t0. Next, an operating section 15 of the body 10 is operated. The operation marks time point t1.
As illustrated by a curve in FIG. 5, since the stylus 11 is stationary from t0 to t1, the displacement remains at y0. When the operating section 15 of the body 10 is operated at t1, force is also transmitted to the body 10 integrated with the operating section 15 to vibrate the body 10. Since the encoder 12 detects a relative displacement of the stylus 11 with respect to the body 10, the vibration of the body 10 is sensed as a displacement of the stylus 11.
Although the measurement reference value is stored in response to the operation of the operating section 15, the storing takes place at time point t2, slightly later than time point t1 at which the operating section 15 has been operated. Since t2 is immediately after the operation of the operating section 15, the body 10 is vibrating considerably The displacement of the stylus 11 stored as the measurement reference value is y1, which is different from the displacement y0 at an actually stationary position. Therefore, the stored measurement reference value includes an error equal to y1−y0.
A well-known way to prevent such an error in the measurement reference value is to provide a time lag between the operation of the operating section 15 and the storing of the measurement reference value.
FIG. 6 illustrates the displacement of the stylus 11, the operation state of the operating section 15, and the timing of storing the measurement reference value in case of the preset operation with time lags.
FIG. 6 differs from FIG. 5 only in that a considerable time lag T is provided between t1 and t2. Since the vibration of the body 10 has settled at time point t2 after a considerable time lag T, the displacement y0 at the actually stationary position can be stored as a measurement reference value.
However, according to such a way of providing a time lag, the time lag needs to be determined longer than the time it takes for the vibration of the body 10 to settle. Since the vibration of the body 10 varies in accordance with, for example, an environmental condition, there are times when the time lag T is too long with respect to a vibration continuation time T0, as shown in FIG. 6. In this case, time equivalent to T−T0 is wasted, which is not preferable in terms of work efficiency.
On the other hand, as shown in FIG. 7, there are cases in which the time lag T is too short with respect to the vibration continuation time T0. In this case, since the vibration of the body 10 is not settled at time point t2, the displacement of the stylus 11 stored as the measurement reference value is y2, which is different from the displacement y0 at the actually stationary position. An error equal to y2−y0 is inevitable.
Such an error in the measurement reference value has been found not only with test indicators but also with various measuring tools whose operating section for controlling a preset operation is provided on a body.